Optical disks such as compact disks (CDs), video compact disks (VCDs) and digital versatile disk (DVDs) are able to be played by recording and reproducing apparatuses. When an optical pickup head of an optical disk drive operates, the light emitted by a light source such as a laser diode is focused by an object lens of the optical pickup head on an optical disk, and the light reflected by the optical disk is transmitted to a light sensor to realize information from the disk. Referring to FIG. 1, the optical pickup head 10 moves along two main directions, i.e. a direction perpendicular to the disk face, referred as a focusing direction F, and a direction parallel to the disk face, referred as a tracking direction T.
During operation, a focusing error and a tracking error usually arise on the rotating disk. The focusing error is generally caused by vibration of the disk in the focusing direction F, and the tracking error is caused by eccentricity of the disk in the tracking direction T. To correct these errors, a focus control system for an optical pickup head was developed, as can be seen in FIG. 2. The optical pickup head 20 has six light receiving parts A, B, C, D, E and F for producing output signals a, b, c, d, e and f, respectively, when receiving light reflected from the optical disk. The output signals e and f are amplified through a first pre-amplifier 21 to produce a tracking error signal TE. A difference between the summation of signals a and c and that of signals b and d, i.e. (a+c)−(b+d), is amplified through a second pre-amplifier 22 to produce a focusing error signal FE. The focusing error FE is processed by a digital signal processor (DSP) 23 to generate a focus control signal Foo to an actuator 24. The actuator 24 provides a moving force for actuating a trace shift of the optical pickup head 20 in the tracking direction accordingly.
The digital signal processor 23 includes a first compensator 231 and a second compensator 232 for complying with various operational statuses. The first compensator 231 has a normal gain, while the second compensator 232 has a high gain. When the optical pickup head 20 has completed a focusing operation and is situated in either a tracking condition or a seeking condition, the first compensator 231 with the normal gain is selected for generating the focus control signal Foo in order to maintain the focusing result. The term “tracking condition” used herein means that the position of the optical pickup head with respect to the tracks is maintained in the proper center position above the selected track. The term “seeking condition” means that the optical pickup head jumps from one track to another track. On the other hand, before the focusing operation of the optical pickup head 20 is completed, the second compensator 232 with the high gain is selected for generating the focus control signal Foo in order to speed up the focusing operation. Such focus control strategy essentially provides good focusing operation of the optical pickup head 20. Unfortunately, in practice, when the optical pickup head 20 has completed a focusing operation and is situated in the seeking condition, the focusing error signal FE is usually unstable due to the production of noise. Therefore, the first compensator 231 is insufficient to provide an appropriate tuning, which becomes problematic in a high-speed and high-precision optical disk.